"T"'s comes and goes because there are 12 different leads (electrode, transducers pasted on the body). The leads have a different orientation to the heart and it traces the electrical impulse as it is transmitted through the body.It is normally inverted in lead R. Ts are variable in the other leads (III, L, F, and V1-V2).
But Inverted T waves may be seen in both ischemia and infarction, late in pericarditis, ventricular hypertrophy, bundle branch block, and cerebral disease.